In present-day copying machines, which offer double-sided and other modes of copying, it is often necessary that the sheets of copy paper change directions several times within the machine. Such changes in direction as well as mechanical tolerances, often result in the sheet falling outside the preferred zone of positionment, defined by an ideal transport line. A device for conveying and positioning sheets is described in U.S. Pat. No. 1,951,901, wherein the positioning means comprise a pair of rollers for moving the sheets perpendicularly relative to the direction of transport and a detection device on the ideal transport line for one of the longitudinal sides of the sheets. After the leading edge of a sheet has been aligned against a number of abutments, the pair of positioning rollers is brought into the operative position to move the sheet perpendicularly to the initial direction of transport. As soon as the detection device detects one of the longitudinal sides of the sheet, the pair of positioning rollers is set to the inoperative position via a control circuit so that the movement is stopped. The disadvantage of this device is that it is possible to position the sheet to the ideal transport path from only one direction. When the sheet supplied has one of the longitudinal sides already past the ideal transport line for that longitudinal side (relative to the transport path), it is no longer possible to correct this deviation using the positioning device described in U.S. Pat. No. 1,951,901.
In addition, positioning sheets by reference to a detected position is not satisfactorily accurate, since fluctuations in switching of positioning rollers into and out of operation, response times in the control system, and mass inertia, both of the components and the sheet, cause inaccuracies.
Another example of prior sheet aligning mechanisms is described in U.S. Pat. No. 3,240,487. This device, however, does not provide a means for positioning one side of a sheet along an ideal transport line. Similar devices include Japanese Patent Application Nos. 63-171748 and 63-171749. It would be desirable, therefore, to provide a device for conveying and positioning a sheet that does not have the above-described disadvantages, and permits positioning of a sheet that strays to either side of the ideal transport line.